Wow. Well, the answer is both simple, and extremely complicated at the same time.<br /><br />Think of the ecosystem as a giant machine, with each life-form as a cog in the machine. Now, there is such redundancy built into the system, that in many situations, the fluctuation of whether cog A is primarily powered by cog B or cog C only affects at a localized level, and doesn't cascade and breakdown the system.<br /><br />Certain cogs (species) have a much greater impact, however. Either more cogs are attached to theirs, or one specific cog is-- and this is where a cascade effect occurs. Mind you, just because you have a cascade effect doesn't mean the system breaks down and stops functioning-- it means that the system AS IT CURRENTLY WORKS breaks down. Prime example-- the one-two punch of the decrease in amphibians and the white nose disease killing off bats in northeastern USA. Less frogs, more insects. Less bats, more insects. It hasn't broken down the system, but instead has shifted it more favorably to bugs. We haven't seen an increase in the other insect eating populations yet, however, to compensate. We may not, even though there is an abundance of food, because the stable balance of the system has not been determined yet by nature itself. However, this doesn't mean the system is broken-- it means the new paradigm for the system is favorable to insects.<br /><br />I'm not sure what you are exactly asking for the detail part of the question-- but human activity that violates the eco-system range from destruction of the eco-system (that NO LIFE benefits) to the bizarre concept of "managing" an eco-system. The reason I state that wildlife management is a bizarre concept is that it supposes that a STATIC eco-system is not only obtainable, but sustainable... which is completely delusional outside of controlled laboratory settings.